[PATCH, libgcc, ARM] __gnu_f2h_internal inaccuracy

[John Tytgat <john at bass-software dot com>]

__gnu_f2h_internal in libgcc converts single-precision floating-point value
to half-precision value for ARM.  I noticed there is a slight inaccuracy
for floating-point values 2^-25 + epsilon (with epsilon > 0 and < 2^-26).
Those should all be converted to 2^-24 in half-precision.

And this because the mask used to implement the even-odd rounding for
aexp = -25 is wrong.  Currently we have:

  /* Decimal point between bits 22 and 23.  */
  mantissa |= 0x00800000;
  if (aexp < -14)
    {
      mask = 0x007fffff;
      if (aexp < -25)
	aexp = -26;
      else if (aexp != -25)
	mask >>= 24 + aexp;
    }
  else
    mask = 0x00001fff;

But when aexp is 25 the mask should be 0xffffff instead of 0x7fffff as the
decimal dot in half-precision will be between bit 24 and 23 of the
above mentioned mantissa.  Cfr. the even-odd rounding done:

  /* Round.  */
  if (mantissa & mask)
    {
      increment = (mask + 1) >> 1;
      if ((mantissa & mask) == increment)
	increment = mantissa & (increment << 1);
      mantissa += increment;
      if (mantissa >= 0x01000000)
       	{
	  mantissa >>= 1;
	  aexp++;
	}
    }

Attached patch solves this problem.  I've left out the clamping of
aexp to -26 for values less than -25 as this it not necessary.  After
the even-odd rounding all aexp values less than -24 will result in +0. or
-0. anyway.

John Tytgat  <John@bass-software.com>

	* config/arm/fp16.c (__gnu_f2h_internal): Fix inaccuracy.

I've got a copyright assignment but no write access.

John Tytgat.
-- 
John Tytgat                                                             BASS
John@bass-software.com